1. Field of the Invention
The present invention relates to improvements in actuator control in an optical disk apparatus, for controlling the position of the optical head for performing recording and reproduction of signals to and from optical disks.
2. Description of the Related Art
Optically recording and reproducing information to and from an optical disk using a light source such as a laser requires that focus control is performed in order to appropriately position the focal point (convergence point) of the light beam onto a recording surface of the optical disk and that tracking control is performed so that the light beam follows the tracks on that recording surface. In particular, when a recording density is high and so the spot diameter of the light beam should be small, the permissible defocus amount and detrack amount also become small, creating a need for more precise control.
In a conventional optical disk apparatus, an observer has been introduced as one way to improve control performance (see JP 2003-141754A (pg. 3, FIG. 7), for example).
FIG. 7 is a block diagram showing the configuration of the optical disk apparatus set forth in JP 2003-141754A. Reference numeral 21 denotes an optical disk to and from which information is recorded and reproduced by a light beam irradiated from an optical head 22. Here, FIG. 7 shows only the circuit for performing tracking control of the light beam, and the circuits involved in recording and reproducing information signals have been omitted from the drawing. The position of the objective lens (not shown) of the optical head 22 is controlled by driving via an actuator coil 23 such that the light beam is appropriately positioned on the recording surface of the optical disk 21. Light reflected from the optical disk 21 is converted into an electrical signal by a photodetector (not shown) of the optical head 22, reproducing the information signal. Along with this, a tracking error signal is generated by a tracking error signal generation circuit 24 based on the signal that has been converted to the electrical signal. The tracking error signal is supplied to a PID control circuit 25 and used for tracking control. The PID control circuit 25 has a configuration that is used for ordinary actuator control, and includes a low-frequency-region compensation circuit 25A, a proportional computation circuit 25B, and a phase compensation circuit 25C.
A disturbance estimation observer 26 receives the output of the tracking error signal generation circuit 24 and the output of the PID control circuit 25 as input and estimates the disturbance that is added to the actuator coil 23. A summing circuit 27 takes the sum of the output of the PID control circuit 25 and the output of the disturbance estimation observer 26 and supplies the result to a drive circuit 28. The drive circuit 28 drives the actuator coil 23 in correspondence with the output of the summing circuit 27.
The overall system of this optical disk apparatus is controlled by a system controller 29. A state determination circuit 30 determines the transient time of the tracking pull-in operation based on an operation command output by the system controller 29 and the tracking error signal output by the tracking error signal generation circuit 24. The disturbance estimation observer 26 is switched on or off depending on the output of the state determination circuit 30.
The disturbance estimation observer 26 for example has the configuration shown in FIG. 8. As one input of the disturbance estimation observer 26, an input voltage (Vi) of the PID control circuit 25 is input to the V→X conversion circuit 31. As its other input, a voltage (Vo) output by the PID control circuit 25 is input to a V→F conversion circuit 32. The V→X conversion circuit 31 converts the input voltage (Vi) of the PID control circuit 25 into a displacement amount (X) of the actuator 23. The V→F conversion circuit 32 converts the output voltage (Vo) of the PID control circuit 25 into a drive force (F) of the actuator 23. The displacement amount (X) and the drive force (F) are input to an estimating circuit 33, and from this the estimated disturbance is output and supplied to a F→V conversion circuit 34. The F→V conversion circuit 34 performs the opposite conversion to the V→F conversion circuit 32, converting the drive force (F) of the actuator into the voltage value output by the PID control circuit 25.
“Mn” in the estimating circuit 33 denotes the nominal value of the lens mass supported by the actuator 23, and “s” denotes a Laplacian operator. “g1” and “g2” are coefficients determining the properties of the disturbance estimation observer 26. Further, (1/Mn·1/s·1/s) represents the model of a secondary resonance-type actuator.
With this optical disk apparatus, the PID control circuit 25 and the disturbance estimation observer 26 both are in operation during tracking control. Further, the disturbance estimation observer 26 uses the input and the output of the PID control circuit 25 to estimate the disturbance that is added to the actuator 23, thereby allowing the drive signal from the PID control circuit 25 to be corrected accurately and allowing disturbance to be inhibited efficiently.
The tracking pull-in operation of the conventional optical disk apparatus configured as above is described with reference to the waveform diagram of FIG. 9. (a) shows the system control command (“command” hereinafter) output from the system controller 29. TrOFF is the tracking “off” command, and TrON is the tracking “on” command. A waveform (b) shows the output of the result determined by the state determination circuit 30. A waveform (c) is the tracking error signal, (d) is the integral value of the PID control circuit 25, (e) is the integral value of the disturbance estimation observer 26, and (f) is the drive signal output from the drive circuit 28. As indicated by the output of the state determination in (b), the waveforms prior to the temporal point t1 indicate the tracking “off” state, during the period of t1 to t2 indicate the transient state of the tracking pull-in operation, and after t2 indicate a steady control state in which the tracking operation has stabilized.
In the tracking “off” state, when the tracking “on” command (a) is output from the system controller 29, then the PID control circuit 25 generates a control signal from the tracking error signal (c) that is input at that time and performs control of the tracking pull-in operation. As a result, the actuator coil 23 is driven by the drive signal (f) from the drive circuit 28 and the amplitude of the tracking error signal (c), which until that point had been output as a sinusoidal wave pattern, becomes smaller with the progress of the pull-in operation. At this time the state determination circuit 30 determines that the pull-in operation is occurring based on the command (a) and the action of the tracking error signal (c), and keeps the disturbance estimation observer 26 in the “off” state as shown in (e).
When the tracking error signal (c) has stabilized at a center value and it has been detected that the off-track amount has settled to within a predetermined range, the state determination circuit 30 makes the determination that the tracking operation has become steady, and activates the disturbance estimation observer 26. The disturbance estimation observer 26 accordingly then performs disturbance estimation using the tracking error signal (c) near the zero-cross point from the start of processing.
When the tracking pull-in operation first starts, the tracking error signal (c) fluctuates considerably and thus it cannot be said to represent the movement of the actuator 23 accurately. Using the tracking error signal (c) at this point therefore would lower the accuracy of the disturbance estimation. On the other hand, the tracking error signal (c) near the zero-cross point after the tracking operation has stabilized faithfully represents the movement of the actuator 23, and thus allows more accurate disturbance estimation to be performed. Consequently, setting the disturbance estimation observer 26 to “off” as mentioned above eliminates the factor of the disturbance estimation observer 26 being unstable during the pull-in operation, and by adding a control that employs disturbance estimation in the steady control state, the stability of the operation is increased.
As illustrated above, the state determination circuit 30 determines the control state from the command and the tracking error signal, and by detecting that there has been a transition from the pull-in state to the steady state and activating the disturbance estimation observer 26, the disturbance estimation observer 26 can perform disturbance estimation using accurate information from the moment that it is activated. This makes it possible to prevent the control from becoming unstable due to a large error in the disturbance estimation.
However, although the above optical disk apparatus can improve the stability of the pull-in operation, control becomes unstable when there is considerable fluctuation in the gain crossover frequency, such as at the time of mode transition, during which the focal point position is moved between a plurality of recording surfaces. Further, the estimation of the disturbance, etc., becomes inaccurate and causes the error control to become unstable if in the steady state there is considerable fluctuation in the error signal due to external vibration that causes the relation between the error signal and the position of the actuator coil to be lost.
In other words, the observer processing that is introduced in order to ensure that control is stable in the steady state has the opposite effect of causing the control to become unstable at the time of mode transition or if a disturbance or vibration is added.